Although you're not allowed to climb it because it is 'very active' the Arenal wasn't that active at the end of April '09, (only saw some lava rocks rolling down at night) but I had a nice walk through the forest at its bottom with plenty of wildlife.

Not actually a "summit," they don't actually allow hiking all the way up volcano that's still shooting out fiery lava rocks... but a pleasant rainforest hike along the slopes, with monkeys, etc. to a clearing where, after waiting about 15-30 minutes, we started to see the fiery rocks tumbling down the side of the volcano... very cool, one of the must-sees in Costa Rica.

"So I was sitting in my cubicle today, and I realized, ever since I started working, every single day of my life has been worse than the day before it. So that means that every single day that you see me, that's on the worst day of my life."
--Peter Gibbons (Office Space)